How do I sync between iOS devices on the same iCloud account?

Note: You must have your HomeKit accessory paired to a master iOS device before syncing between iOS devices on the same iCloud account.

To enable iCloud syncing of your HomeKit data, you will need to enable iCloud key chain on your iOS devices using your iCloud account if you haven't already done so.

To enable iCloud Keychain go to Settings > iCloud > Keychain and enable "iCloud Keychain" and then follow the onscreen instructionsNote: Enable iCloud keychain on the master iOS device before enabling it on other iOS devices

Sign in using your iCloud account on the other iOS device and enable iCloud key chain

iCloud syncing of HomeKit data can take 2-24 hours. If you can see your HomeKit accessory from your secondary iOS device(s) then the iCloud sync has been completed.

Troubleshooting:

Make sure your iOS devices are using iOS 9 or higher

In some scenarios, iCloud syncing has better results if your Apple ID ends in @icloud.com. If you are using a third party email address, you might be prompted to set up an alternate Apple ID ending with @icloud.com.